1. Field of the Invention
The present invention relates generally to methods for fabricating semiconductor devices and, more particularly, to methods for forming a photoresist pattern using an anti-optical proximity effect.
2. Description of Related Art
Photolithography is a common process or technique employed in the manufacture of semiconductor devices. Typically, a substrate or wafer is coated with a layer of light-sensitive material, such as a photoresist. Using a patterned photomask, the wafer is exposed to light that manifests a photochemical effect on the photoresist to produce a photoresist pattern, which is printed onto the photoresist.
The size and shape of the photoresist pattern may be different from the patterned photomask due to an optical proximity effect (OPE). The excessively rounded corners generally can lead to problems such as line shortening in which the length of the photoresist pattern is shortened. In addition, the excessively rounded corners may make it difficult to control the critical dimension of the photoresist pattern. Consequently, the quality and the yield of the semiconductor devices can be deteriorated.
One method of reducing excessively rounded corners is to perform multiple photolithography processes along with multiple etching processes to produce a rectangular photoresist pattern. This method, however, is more complicated and can require a greater amount of time and process steps to produce the resulting semiconductor device.
Another method of reducing excessively rounded corners is to provide an optical proximity correction (OPC) or an anti-OPE. The OPC is provided where the photoresist pattern printed on the photomask is distorted, and provides a predistortion at such locations in a direction opposite to a direction of curvature of the lens. Photomasks having such distorted patterns are known as optical proximity correction masks (OPCMs). However, even though the excessively rounded corners may be reduced using the OPC, the formed pattern still may not optimally form the desired shape, such as rectangle.
FIG. 1A is a top view of a prior art photomask 10 without OPC wherein main patterns 12 are used to block the light and are formed on a transparent photomask plate 14. FIG. 1B is an exploded top view of one of the main patterns 12 of FIG. 1A with the shape of a pattern 16 printed on a wafer 18 (see FIG. 1C) and superimposed on the main pattern 12. As shown in FIG. 1B, the shape of the main pattern 12 of the photomask 10 is a rectangle having sharp corners. However, the pattern 16, which is actually printed onto the wafer 18, has excessively rounded corners due to the OPE. In the three-dimensional view of FIG. 1C, the corners of the wafer 18 are excessively rounded, similarly to the excessively rounded corners in the pattern 16, due to the OPE. When the corners are excessively rounded as in the pattern 16, the length and width of the resulting structure formed in the wafer 18 may be shortened, thereby diminishing the quality and reliability of the semiconductor device. Accordingly, an OPC mask, an example of which is shown in FIG. 2A, may be utilized to attenuate some of the drawbacks described above.
In an attempt to alleviate the excessively rounded corners caused by the OPE, a number of subsidiary patterns 20 may be formed on the transparent photomask plate 14 to overlap each corner of the main pattern 12 to generate an improved prior art photomask 22, as shown in FIG. 2A. In particular, each main pattern 12 is combined with four subsidiary patterns 20 to thereby produce an improved pattern 24. Each subsidiary pattern 20 outwardly distorts from the edge of each corner of the main pattern 12 to compensate for the effect that when printing the pattern 16 onto the wafer 18, the shape of the printed pattern 16 has excessively rounded corners, thus being inwardly distorted compared to the main pattern 12. The corner rounding error of the pattern 16 (FIG. 1B) printed on the wafer 18 can be improved when using the improved pattern 24. FIG. 2B is an exploded top view of one of the improved patterns 24 of FIG. 2A with the shape of a pattern 26 being printed on the wafer 18 and superimposed on the improved pattern 24. Using the pattern 26 as a photomask, the wafer 18 is developed to form a three dimensional structure on the wafer 18, which is shown in FIG. 2C. The improved pattern 24 still does not produce a pattern 26 that has a rectangular shape. As shown in FIGS. 2B and 2C, the width and length of the pattern 26 can be wider and longer than that of the main pattern 12; thus the pattern 26 may be outwardly formed larger than the main pattern 12, causing what is called an under- or less-shoot. Furthermore, the corners of the pattern 26 are still excessively rounded.
A need thus exists in the prior art to develop methods for reducing the excessively rounded corners caused by the OPE. A further need exists to develop methods that can correct the over-shoot problem.